The proposed research will evaluate methionine and selenomethionine, cysteine and selenocysteine as substrates for the corresponding aminoacyl-tRNA synthetases from a variety of plants; plants will include selenium-accumulating species and nonaccumulating species. Cell-free protein synthesizing systems will be developed from selenium-accumulating and nonaccumulating plant species and used to study the incorporation of selenomethionine and selenocysteine into protein. 75Se-labeled plant proteins will be treated with N-ethylmaleimide and iodoacetic acid to protect selenol groups, and the protein hydrolysates will be analyzed for the N-ethylmaleimide and carboxymethyl derivatives.